1. Field of the Invention
This invention relates to circuit interrupters and more particularly to a circuit interrupter for distribution transformers to control moderate power distribution on feeder circuits.
2. Description of the Prior Art
Transformers used in power distribution systems are generally associated with a protective device which prevents or limits current overload damage to the transformer and its associated apparatus. A completely self-protected transformer includes a circuit breaker on the secondary or low voltage side to protect against damage due to overload currents. The secondary circuit breaker disconnects the transformer from its load if the load current becomes dangerously high.
A problem exists in some of the prior art transformer circuit breakers which use plastic members for the operating and/or support of the circuit breaker. Holding the many dimensions with associated close tolerances requires extremely accurate molding. The hot oil environment in which the circuit breaker must function accurately is less than ideal for even the best plastic. The retention of the calibration during transformer processing and while in service depends to a large extent on the plastic materials maintaining their specific dimensions. For accuracy and cost of construction it is desirable that the number of movable plastic components in the circuit interrupter be minimized.
In prior art circuit breakers a flexible lead is required entering the circuit breaker at the stationary contact to provide for contact movement during contact closure. A flexible lead is also required between the moving contact and the bimetal, and another flexible lead is required to exit the breaker from the other end of the bimetal to allow for movement of the bimetal assembly during calibration and when the breaker is reset. The extensive use of copper braid, with the many brazes required, provides a circuit breaker which is difficult to calibrate and expensive to manufacture. It is desirable to have a circuit interrupter constructed to minimize the amount of flexible braid required.
Commonly used circuit interrupters incorporated bimetal thermal trip and instantaneous magnetic trip. For overload current it is desirable that circuit interruption be completed rapidly after initiation.